1. Field of the Invention
The present invention relates to a gas sensor herein a central electrode is passed through a coil serving as both a heater and an electrode and the coil is buried in a metal oxide semiconductor bead, and to a method of production thereof
2. Prior Art
The present applicant proposed a gas sensor wherein a central electrode is passed through a coil serving as both a heater and an electrode and they are buried in a bead of a metal oxide semiconductor and one end of the central electrode and both ends of the coil are attached to pins to provide three-point support to the bead (Japanese Patent Opening Sho 61-264246). Such a sensor, however, has an insufficient mechanical strength because of its three-point support, and it is difficult to position accurately the central electrode in relation to the coil.
3. Summary of the Invention
The object of the present invention is to provide a gas sensor of which mechanical strength is high and of which dispersions of characteristics such as sensor resistance are small, and a method of production thereof.
According to the present invention, a metal oxide semiconductor bead in which a coil and a central electrode are buried is supported above a concave provided in a base, and both ends of the coil are attached to two of three leads of the base. Now, if the central electrode can be positioned in the center of the coil, dispersions of resistance, etc. of the gas sensor will be reduced. The central electrode can be positioned by, for example, guiding both ends thereof out of the base and using chucks, etc. Alternatively, one end of the central electrode can be positioned by the remaining central lead of the three leads, and the other end thereof can be positioned by holding it above the concave or outside the base. Thus dispersions of the gas sensor characteristics can be reduced.
One end of the central electrode is fixed on the lead, and the other end thereof is fixed on the surface of the base or in the concave or on another lead. Accordingly, the sensor element is provided with four-point support, and this support significantly increases the mechanical strength in comparison with the three-point support.
In molding the base, the three leads are provided in plate forms, they are made to penetrate, in parallel with each other, the base within the same plane, the central lead is bent towards the side opposite to the concave, and the other two leads are bent towards the side of the concave. In this way the base having a concave can be molded easily together with the leads. A smaller base can be used by efficiently allotting the principal face of the base to an area for locating the three bent leads and an area for the concave.
If the concave is extended to one side face of the base and the other end of the central electrode is attached to the bottom of the base, handling of the central electrode can be facilitated and a larger concave can be provided. Moreover, as the central electrode bends between the bottom of the concave and the coil, some slack will be generated in its length, and this in turn will increase the resistance of the gas sensor to impacts of fall, etc.
Means for giving some slack to the central electrode are not limited to attachment of one end of the central electrode to the bottom of the concave. Provision of a bend between the attachment and the inside of the coil will do.
According to the production method of the gas sensor of the present invention, both ends of the coil are attached to the leads on both sides of the concave of the base, the central electrode is passed through the coil and positioned in relation to the coil, then a bead is formed. With this arrangement, dispersion of the clearance between the central electrode and the coil can be reduced, and the sensor characteristics can be made more even. Moreover, as both ends of the central electrode are fixed, the sensor element is supported by four points, and its mechanical strength is increased.
As for positioning of the central electrode, for example, both ends of the central electrode are chucked and positioned, or one end thereof is positioned on a lead and the other end is chucked and positioned.
If a thin wire is used for the central electrode to reduce the power consumption, it will be difficult to cut off the central electrode without generating any distortion and to pass the central electrode straight through the coil. Hence the wire of the central electrode is let out of a capillary, and the capillary is made to move forward to pass the wire through the coil. Next, the top end of the wire is held and the capillary is made to retreat so as to let out the wire from the capillary. After that, when the wire is locally melted for cutting the wire off, balls will be generated in the melted portions. Thus the wire can be cut off without generating any distortion in the wire. The wire having a ball at the top end thereof and being free of any distortion can be easily passed straight through the coil. In this way, the wire of the central electrode is passed through the coil, let out of the capillary and provided with a ball at the top end thereof